Biaxially oriented polypropylene (BOPP) films used for packaging applications perform multiple functions. It must perform in a lamination to provide moisture and/or oxygen barrier; it must provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating; often it must also provide a surface suitable for printing to display graphic designs.
U.S. Pat. No. 5,500,282 describes the use of high crystalline content propylene homopolymer with an intermolecular stereoregularity greater than 93% mixed with “a moisture barrier improving amount of polyterpene resin.” This formulation provides an oriented film structure of improved water vapor transmission rate.
U.S. Pat. No. 5,667,902 describes the use of blending a high crystalline propylene homopolymer having an isotactic stereoregularity of greater than about 93%, with a second propylene homopolymer having an isotactic stereoregularity of from about 90% to about 93%, and a resin modifier wherein the resin modifier is hydrogenated hydrocarbon resin.
U.S. Pat. No. 6,503,611 describes the use of blends of propylene-based polymers with crystalline waxes as a cold seal release layer. Release properties are adequate; however, processability is an issue with the low molecular weight waxes causing 1) die build-up issues; 2) smoke generation; 3) tendency to stick to downstream rollers in tentering operations. Also, since the wax is a component of a relatively thin skin layer, there are no substantial improvements in moisture barrier or oxygen barrier properties.
U.S. Pat. No. 6,033,514 describes the use of multilayer biaxially oriented polypropylene films with improved moisture vapor transmission rates by formulating a core resin layer with an amount of crystalline wax. This core layer is then encapsulated by polyolefin cap layers to help prevent blooming of the wax to the surface and causing plate-out issues. However, the cap layers utilized by this patent are non-polar polyolefins and the wax can readily migrate from the core layer into and through the cap layers and onto the surface of the cap layer. Thus, although the rate of wax migration may be more controlled, it still occurs nevertheless, and is still prone to the afore-mentioned plate-out issues.
U.S. Pat. No. 6,033,771 describes the use of waxes to improve moisture and oxygen barrier properties of multilayer BOPP films. In this invention, the wax is blended into a core layer and an intermediate cavitated layer between the core layer and surface layer is used to entrap the wax within its voids and prevent its migration to the surface, thus avoiding plate-out problems. However, the cavitation of the intermediate layer renders said invention opaque and is no longer transparent.
U.S. Pat. No. 5,141,801 describes the use of wax incorporated into a crystalline polyolefin layer for improved moisture barrier properties with an interior layer of EVOH to prevent migration of the wax through the surface. The EVOH also provides oxygen gas barrier properties to the film structure. However, the EVOH layer is susceptible to environmental humidity conditions and consequent loss of barrier properties. Thus, this patent recommends encapsulating the EVOH layer with a second wax-containing polyolefin blend layer to protect the EVOH from moisture diffusion. However, this second wax-containing layer in such multilayer structure means that the wax is free to migrate to its surface and continue to cause plate-out issues.
This invention seeks to provide an economical, transparent, improved moisture barrier BOPP film that avoids some of the disadvantages of the prior art films.